My truck has jsut developed this niggling little problem. I need some help troubleshooting it please!

I have a 1998 5.4L RWD Expedition 155 000 miles. When I bought it I replaced all the COP's the spark plugs and fuel injectors because of a missfire at part throttle. I also replaced the fuel filter and ran a bottle of "fuel system cleaner" for what it was worth.

The truck has run fine for the last few months, but now it is developing an annoying hesitation at part throttle. To be clear this is not a missfire..... I know because I had a missfire in the past. So if I'm on the freeway doing 50 - 70 and I gently press the accelerator there is a slight flat spot there. if I press harder the truck accelerates fine. Oh and its not throwing any error codes.

Going back to when I bought the truck it had a bad missfire. One of the first things I checked to remedy the missfire was the fuel pressure. I can't remember the exact numbers, but I think its supposed to be about 40psi at idle and rise to about 50 when you take the vacum tube off.

Anyway thats not important what is important is that while the fuel pressure tests checked out, I noticed that turning the ignition on, but not starting the motor would increase the fuel pressure by only about 10psi. Turning it off and on again repeatedly would bring the pressure up to the required 40psi. I put this down to a weak fuel pump, but didnt go any further because the tests checked out once the motor was running.

I should also point out that the motor cranks a few times before it fires. Now that this flat spot has developed, the fuel pump is my prime suspect, but I don't know how to test it....... or if I shuold just trump up the $100 or so and replace it anyway.


Any advice would be much apreciated.


Oh one other detail that may be important. When I replced the fuel filter some "mud" poured out of the intake side of the old filter. Should I be rinsing out the gas tank when (if) I replace the fuel pump.

 

Have you replaced the fuel filter? It could be plugged and restricting fuel flow.

 

I replaced the filter about 5,000 miles ago. When I replaced it the intake side was full of dirt.


Could it clog up again that quickly? If so the tank must be full of dirt.... and then where is it comming from??


I think I'll give the filter idea a try this weekend.


Anybody know how to clean the tank out??